As a result of manufacturing precision products in the injection molding field, the injection speed, precision and pressure may be controlled by various devices. An electrical injection molding machine adopts a servomotor that can process feedback control for speed and position, but it cannot detect the injection force. One way to resolve this problem is to install a load cell to measure the axial force of the screw. In the prior arts, due to the arrangement and rigidity of the load cell, it does not make a close loop to force when fastening mechanical parts as is required to correctly measure the real value of the axial force. On the other hand, the friction causes a sluggish condition during injection, such that the injection force and the friction cannot be determined by the load cell simultaneously, thus the axial force during delivery does not compensate for the lost force over time. As the experience form the prior art, the precise injection force measurement, holding pressure and back pressure control for the high-speed and accurate injection are not workable.
Controlling the pressure path of the injection force of the injection molding machine, and knowing a force value accepted by a screw instantly and accurately is very important. The load cell measures the force, and then the controlling system compensates for a difference between the measured force value and a pre-setting value controlling injection, holding pressure and metering back pressure.
Please refer to FIG. 1, which is a first embodiment in prior arts and a patent of 5421,712 of the United States, called “Screw Rotating and Advancing Device for An Injection Molding Machine”. A load cell 39′ arranged on a moving plate 23′ accepts an axial force that a protruding collar 471′ of a connecting block 47′ applying on a thrust bearing 45′. Therefore, the axial force is transferred to a strain voltage value by the load cell 39′. Further the strain voltage value is inputted into a control system (not shown in the figure) via an load cell amplifier (not shown in the figure). The control system corrects an output of an injection servomotor (not shown in the figure) according to aforesaid difference in the previous paragraph for appropriately controlling injection force and approaching a purpose of pressure control of a close loop.
FIGS. 2A and 2B are two sectional view of the first embodiment in prior arts. FIG. 2A shows a path for the axial force in delivery during injection and holding pressure. Movement between a ball screw 34′ and a ball nut (not shown in the figure and normally positioned above the ball screw 34′) generates an axial pushing force, then passing through a thrust bearing 37′, a bearing housing 371′, a compressing seat 391′, the load cell 39′, a connecting block 48′, the moving plate 23′, continuously to a thrust bearing 45′, the protruding collar 471′, a half ring 171′ and the screw 17′, shown as a line A. Partial force transmits to the compressing seat 391′ and then to the connecting block 48′, shown as a line B. Other partial force is to moving plate 23′, shown as a line C, then both are to the connecting block 47′ and the screw 17′ via the thrust bearing 45′ and a bearing compressing plate 46′.
FIG. 2B shows a path of metering back pressure. For a line A′, partial force transmits to load cell 39′ via the half ring 171′, connecting block 47′ and thrust bearing 45′. Most other force transmits to connecting block 48′ by way of the bearing compressing plate 46′ or thrust bearing 45′, shown as a line B′, and to moving plate 23′, shown as line C′. Aforesaid three force lines are combined at load cell 39′, and a combined force goes to ball screw 34′ through compressing seat 391′, bearing housing 371′ and thrust bearing 37′.
No matter what the condition is in injection, holding pressure (shown as FIG. 2A_or metering back pressure (shown as FIG. 2B), the force can not be delivered to load cell 39′ precisely and accurately because of many other branch forces, it derives a result that the measured injection pressure, holding pressure and metering back pressure are not exactly same as the realism of screw 17′ accepted. When injection or metering is in execution, the friction from moving plate 23′ and a guiding rod 22′ is variable with the moving speed of moving plate 23′, lubrication degree and other environmental factors. The uncertainty cannot be neglected. to reimburse the disadvantageous may well be the way to approach the purpose of precision injection. In the patent of U.S. Pat. No. 5,421,712, the measured injection force and friction force of load cell 39′ are unreal to cause insufficient signal for compensating injection force. Therefore the control of injection force, holding pressure and metering back pressure is barely made.
Please refer to FIG. 3, which is a second embodiment in prior arts and a load cell arrangement of a patent of U.S. Pat. No. 5,955,117. Firstly, bolts 34″ fasten a load cell 33″ and a ball nut 31″. Secondly, bolts 32″ fasten the load cell 33″ on a rear plate 3″. While a ball screw 28″ is axially rotating, the rotation is a relative motion with the ball nut 31″ on the rear plate 3″. When the ball screw 28′ rotates and moves toward a left, ball nut 31″ is received an opposite force, which is toward a right. The reason to build up the motion is that ball nut 31″ contacts with a left internal surface of load cell 33″, the force is then transmitted to load cell 33″. Referring to FIG. 4, a left external side of load cell 33″ is bounded by bolts 32″. FIG. 4 is a view of force analysis of the load cell of the second embodiment in prior arts. Load cell 33″ is applied by two forces, one is the trusting force X of ball nut 31′, another is the tensile force X′ of bolts 32′. Due to the deformation of the bolts causing a difference and sluggishness of a trusting force X and a tensile force X′, the measured axial force is not a true value.
Through many years experience in manufacture, devoted study, continuous research, experimental analysis, and improvement, the inventor finally proposes an invention that can reasonably and effectively improve the shortcomings of the prior arts.